1. Field of the Invention
This invention generally relates to fluid sampling. Here, the present invention provides a method and apparatus for trapping and injecting a fluid sample (liquid or gas) at high pressure and temperature, into a chromatograph with no sample loss.
2. Background Information
In petroleum reservoirs, liquids and gases exist under high-pressure, high-temperature conditions. To observe the properties of these fluids in the laboratory, all samples must remain in this condition to truly reflect their natural state. Here, a petroleum sample is taken from the reservoir and brought to the surface where it is subjected to a lower temperature and pressure. However, once in the laboratory, the sample is returned to a single phase by placing it in a device to repressurize and reheat it. (See U.S. Pat. No. 4,425,810 to Simon et al). In the high-pressure visual cell of U.S. Pat. No. 4,425,810, measurements are made either visually or by other optical means. Thereafter, hydrocarbon analyses must be made (usually in a gas chromatograph) so that a sample must be removed from the high-pressure, temperature environment to one of low pressure and temperature.
Some techniques which analyze samples from a high-pressure cell simply bleed off the sample into the low-pressure environment of the gas chromatograph. However, this causes problems. The single-phase mixture of hydrocarbons may separate into two phases, (i.e., gas and oil) which would affect the analyses. Or, there may simply be time delays. For example, bleeding off a sample requires cleaning up the entire cell, putting a new sample in the cell, and repressurizing the system, which itself can take up to one day of labor.
For these reasons, it is important to take a micro sample and keep it under the same high-pressure, high-temperature conditions that exist in the cell. When these conditions are maintained, the hydrocarbon mixture will not separate into two phases. Furthermore, due to the size of the micro sample, the addition of another sample in the cell would be unnecessary.
Several valves have been designed that take micro samples of a fluid at a high temperature and pressure. See Lyman Yarborough and John Vogel, "A New System for Obtaining Vapor and Liquid Sample Analyses to Facilitate the Study of Multi-Component Mixtures at Elevated Pressures", 81 Chem. Eng. Prog. Symp. Series-Phase Equilibria and Related Properties 1 (1967); and D. Legret et al, "Vapor Liquid Equilibria Up to 100 MPa: A New Apparatus", 27 AIChE Jour. 203 (March, 1981). Both show valves that will take such a sample. However, the valve of Yarborough et al has a problem in that the sample is trapped in a cavity that is out of the flow path that leads to the chromatograph. Since the micro sample is placed in a "dead space", all of the components of the hydrocarbon sample may not be properly analyzed because they may not be flushed through the valve. The Legret valve suffers from the same problem as the sample is trapped in an area that is not directly in the flow path to the chromatograph which means that a portion of the sample may be left behind and not analyzed. Furthermore, the Legret valve is used to sample hydrocarbons that have a high boiling point and it consequently does not have the same versatility as the present invention.
The present invention seeks to eliminate the problems encountered in analyzing a high-pressure, high-temperature fluid sample and also to overcome the problems inherent in the Yarborough and Legret valves. To overcome those disadvantages, the present invention brings the micro-sample into the carrier gas rather than having the sample expand into the carrier gas from a dead space. Due to this procedure, the present invention may trap, for analysis, oils that are highly volatile where the other valves may not.